Biomedical Engineering Reference
In-Depth Information
Organic scintillators are available in a variety of forms. Anthracene and stilbene
are the most common organic crystalline scintillators, anthracene having the high-
est efficiency of any organic material. Organic scintillators can be polymerized
into plastics. Liquid scintillators (e.g., xylene, toluene) are often used and are prac-
tical when large volumes are required. Radioactive samples can be dissolved or
suspended in them for high-efficiency counting. Liquid scintillators are especially
suited for measuring soft beta rays, such as those from
14
Cor
3
H. High-
Z
elements
(e.g., lead or tin) are sometimes added to organic scintillator materials to achieve
greater photoelectric conversion, but usually at the cost of decreased efficiency.
Compared with inorganic scintillators, organic materials have much faster re-
sponse, but generally yield less light. Because of their low-
Z
constituents, there
are little or no photoelectric peaks in gamma-ray pulse-height spectra without the
addition of high-
Z
elements. Organic scintillators are generally most useful for
measuring alpha and beta rays and for detecting fast neutrons through the recoil
protons produced.
Inorganic Scintillators
Inorganic scintillator crystals are made with small amounts of activator impuri-
ties to increase the fluorescence efficiency and to produce photons in the visible
region. As shown in Fig. 10.28, the crystal is characterized by valence and con-
duction bands, as described in Section 10.2. The activator provides electron energy
levels in the forbidden gap of the pure crystal. When a charged particle interacts
with the crystal, it promotes electrons from the valence band into the conduction
band, leaving behind positively charged holes. A hole can drift to an activator site
and ionize it. An electron can then drop into the ionized site and form an excited
neutral impurity complex, which then decays with the emission of a visible pho-
ton. Because the photon energies are less than the width of the forbidden gap, the
crystal does not absorb them.
Fig. 10.28
Energy-level diagram for activated crystal scintillator.
Because the energy levels of the activator complex are in the
forbidden gap, the crystal is transparent at the fluorescent
photon energies
h
ν
.
